The early hours of 8 September saw billionaire Glencore chief executive Ivan Glasenberg shuffle out of a Claridges hotel room and head off for a much-needed drink. He had just met Sheikh Hamad bin Jassim bin Jabr al-Thani – Qatar's prime minister, known to City dealmakers as HBJ – and his planned takeover of FTSE 100 miner Xstrata was back on. Still, despite what appeared a triumph, Glasenberg's emotions were mixed.

The Qatari had so disliked the original terms of Glencore's takeover that he had quadrupled his Xstrata stake and been threatening to scupper the merger. But he sensed Glencore was desperate for a deal – and so it proved. The man often referred to as the world's shrewdest commodity trader was forced to buckle and hand HBJ improved terms. The arch trader had been outsmarted.

That meeting will forever be known for a cameo role played by Tony Blair, even though there is disagreement among those close to Glencore and the Qataris about the importance of his role in brokering a deal. But it is also, perhaps, the time when the City really started taking notice of Qatar.

The Square Mile had always known, of course, that the emirate had money to spend, and had courted it. But now everybody saw that it could also play high finance's games, and win them too.

Since then, seemingly every major putative takeover target – plus a few more – have been linked with Qatar. Three weeks ago there was speculation it was to bid for Arsenal, swiftly followed by reports that it was about to launch the "Dream Football League" to challenge Uefa's Champions League. And then, last weekend, there were reheated suggestions that it was considering a £8bn bid for Marks & Spencer, the embattled retailer. Qatar has denied most of that, but it does say it is looking to spend £10bn on British infrastructure over the long term and it clearly has money to invest overseas.

"They have a very substantial annual budget surplus and that's after investing a lot in their own infrastructure and universities and so forth," said one person with close knowledge of the country's operations. "Thirty billion dollars or so per annum is about right. Because of that it is not at all surprising that the first people that anybody thinks of when they scratch heads to think where to get money to invest in a project is the Qataris."

The Qatar Investment Authority, where HBJ is chief executive, is what most people think of when Qatar pops up on a deal, but little is known about its workings. The sovereign wealth fund rankings gives it a "transparency index" of five out of 10 – the same as Iran's sovereign fund – while estimating it has around $115bn of assets.

Even so, the QIA is essentially a holding company and it is its various subsidiaries, mainly Qatar Holding, that have led many of Qatar's foreign acquisitions. It is via QH that Qatar holds its stakes in Barclays, Sainsbury's and German carmakers Volkswagen and Porsche, plus full ownership of Harrods, the luxury department store which houses the QIA's London office.

HBJ is also a major investor behind the Candy and Candy development at One Hyde Park, where penthouse flats could cost £84m.

"I have never met anybody within QH who isn't very, very smart," says one financier close to the sovereign wealth fund. "These people all speak three languages, at least. They are very highly educated. Many of them, like Ahmad Al-Sayed, have been to university or law school in America or the UK. These are clever people. They are very disciplined and rigorous in their approach to evaluating investment opportunities."

Al-Sayed is chief executive of Qatar Holding, which is chaired by HBJ. Deals tend to require the approval of both men, City sources say, but despite their growing reputations, there has been confusion over whether they have any coherent investment strategy. What have luxury flats and Harrods, doubters say, got in common with Xstrata?

But there might be a thread. One person with direct knowledge of the Qataris' investment philosophy said: "In one way, it is quite hard to discern a strategic direction. They don't only invest in metals and mining. They don't only invest in airport operators. They don't only invest in luxury goods and department stores. There is no single asset class which they particularly like.

"I think the thing that particularly defines them as much as anything else is that they have very long-term horizons. They have formulated their views on the basis that the world population has gone past seven billion and they can't think of any reason why it won't go on rising exponentially.

"They look at the phenomenon of the growth of the middle classes around the world and they believe that's going to continue. Population growth plus, within that, growing numbers within the middle classes. They therefore see very strong rises in the middle classes around the world, which means spending power around the world. If a population increases its spending power by that magnitude, then people are also going to buy commodities."

But it is not just the Qataris who need to have long horizons. Those who have dealt with them talk about requiring large reserves of patience in order to get deals through, as the emir is keen to present his country as respectable to foreign business partners. The funds, then, are said to be heavily regulated, and while British compliance officers have looked at the country's investments to check if any involvement with them might prove controversial, most of their conclusions appear bland.

Still, there are moments of controversy. Barclays is currently dealing with allegations about its relationship with Qatar at the height of the 2008 financial crisis, when the emirate was the main contributor to a £7.3bn lifeline that allowed the bank to avoid a taxpayer bailout. The Serious Fraud Office and the Financial Services Authority are reportedly investigating whether Barclays lent Qatar funds to buy shares in the bank.

HBJ also made headlines a decade ago following a £500m arms deal between Qatar and BAE Systems after which £7m was transferred into two Jersey trusts of which he was a beneficiary. The funds were frozen by the Jersey Financial Services Commission, which then began a court case and investigation. HBJ paid the Jersey authorities £6m as a "voluntary reparation" as "the structures put in place by his advisers may have contributed to the cost and complexity of the inquiry". The case was dropped and all parties denied any wrongdoing.

Elsewhere, the old Blair connections consistently re-emerge. The former British prime minister personally persuaded HBJ to contact indebted Irish property investor Patrick McKillen, who was seeking £70m to buy a controlling stake in Maybourne Hotel Group from billionaire brothers David and Frederick Barclay, in an increasingly bitter saga that has ended up with the Irishman fighting the twins in London's high court.

Qatar is thought to have sunk £60m into that venture in an effort to take control of the hotel group that includes Claridges, the venue of the meeting with Glasenberg.

There are those in the City who predict they will get it too – and probably a few more in the Middle East. As one source who has lived and worked in Qatar puts it: "The emir himself is credited with a famous statement. He said, 'I might run the country, but it is HBJ who owns it.'" Increasingly that goes for other parts of the world, too.

Ruling family

The current Qatari emir, Sheikh Hamad bin Khalifa al-Thani, has ruled since 1995 after deposing his father – Sheikh Khalifa bin Hamad bin Abdullah bin Jassim bin Muhammed al-Thani – in a bloodless coup. Legend has it that, having secured support from other factions of the al-Thani family, Sheikh Hamad called his father at a Zurich hotel to inform him. The emir hung up.

Sheikh Hamad has three wives and 24 children, and has named his heir as his second son from his second marriage: Sheikh Tamim bin Hamad al-Thani, who is chairman of the Qatar Investment Authority (QIA). However, many see much of the real power residing with Hamad bin Jassim bin Jabr al-Thani (known as "HBJ"), a cousin of the emir whose position is thought to stem from his support for the 1995 coup.

HBJ is prime minister and foreign minister, chief executive of the QIA, and chairman of its subsidiary Qatar Holding – said to be the emir's personal investment vehicle in a country where personal and state wealth are blurred.

One analyst says: "The current emir is a large man who does not look in perfect health. It is not clear what HBJ's position will be once Tamim is in charge."